Computer Science – Databases
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p43c1702w&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P43C-1702
Computer Science
Databases
[1115] Geochronology / Radioisotope Geochronology, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering
Scientific paper
Knowledge of the impact flux in the early inner solar system is fundamental to understanding the near surface conditions on the terrestrial planets during this formative period. However the lack of preserved >2.0 Ga craters precludes direct sampling of such events over the first 55% of Earth history. We do, however, have preserved crustal materials in the form of detrital zircons with ages up to 4.38 Ga that can potentially act as environmental monitors. In order to interpret zircon growth features in terms of impact events, such as the Late Heavy Bombardment (LHB), we developed a zircon saturation model that estimates the zircon crystallization temperature spectrum and the fraction of Zr in the continental crust expected to be processed through impact melting. Such modeling results can then be compared to the crystallization temperature spectrum of detrital Hadean zircons to estimate the fraction that were impact-produced. Magmatic zircon growth during an impact event is controlled by ambient temperature, Zr content and composition of the target material, and impact energy. Impacts need to be sufficiently large to permit decompression melting of uplifted middle to upper crust (i.e., low energy bolides will not produce melt sheets and thus impact zircons). We modeled the LHB using the thermal model and hypothesized bolide flux of Abramov and Mojzsis (Nature 459, 419, 2009). Target compositions for modern and Archean crust were estimated from large geochemical databases and selected using a Monte Carlo process by which the full spectrum of compositions were randomly chosen. Model results for impact produced zircon from a target of Archean composition yield a zircon crystallization temperature distribution significantly higher than that observed for detrital Hadean zircons from Western Australia. We take this as evidence that impact produced zircons are not a significant source for this population. Modeled results from impacts on modern crust yield temperatures remarkably similar to Ti-in-zircon crystallization temperatures of recent large impact events (i.e., Morokweng, Manicouagan). Refinements of the model include: reduction of sampling bias in compositional databases and development of hypothetical crustal compositions to simulate Hadean crystallization temperatures. This model can be used as an analog to predict zircon abundance and crystallization temperature spectrum for other planetary bodies (i.e. Moon, Mars, Vesta) where surface compositions are known or estimated. When rocks are in hand, in the form of meteorites or from sample return missions, modeled results can be compared to extracted zircons to estimate the contribution of impacts and provide insights into ancient impact histories.
Boehnke P.
Harrison Mark
Schmitt Axel K.
Wielicki Matthew M.
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